SAfter the object is uploaded, the multipart-ness of it does not matter. Is the single PutObject object you're fetching the same size?
SSo I don’t think there’s any correlation between your observations. After you complete a multipart upload, it all becomes one file, it isn’t spread across multiple machines like you’re expecting. It may have more to do with the network speeds you’re getting.
SIs this from your home network?